Where do our teeth come from? [excerpt]

Oxford Handbook of Integrated Dental Biosciences, Second Edition

We all know that we start with baby teeth which fall out and are replaced with adult teeth, but do we really know why? Where do our baby teeth come from in the first place? The science of tooth formation is wrapped up in genetics, and much of it is yet to be understood, but our journey from being toothless babies to adults with a fine set of pearly whites is fascinating. This adapted extract below from the Oxford Handbook of Integrated Dental Biosciences highlights how our teeth form, why they erupt through our gums when they do, what causes teething pains, and when baby teeth should begin to appear.

The early embryo starts as a ball of undifferentiated cells, then organizes into a 3-layered disc:

• Ectoderm—is the origin of skin and infolds to form the nervous system, sensory cells (of eyes, nose, etc), and dental enamel

• Endoderm—is the origin of epithelial lining of gut and respiratory systems, the parenchyma of liver and pancreas

The process of tooth formation starts at 5–6 weeks in utero with the formation of the dental lamina. This is a thickening of the ectoderm extending from the lining of the primitive oral cavity down into the underlying ectomesenchyme. Within this dental lamina, focal bud-like thickenings map out the sites of the future teeth, 20 for the first set of teeth, and later 32 for the permanent teeth. These ectoderm buds, together with a surrounding aggregation of ectomesenchymal cells, form the earliest stage of the tooth germ. There are 6 stages in which the crown of the tooth is formed, which you can see in the diagram. The outer shape of the crown is fully formed before root development starts.

Eruption brings the tooth from its developmental position into its functional position. This mechanism is not fully understood yet. The dental follicle is crucial (it later becomes the periodontal ligament). It seems the tooth is pushed rather than pulled – there does not seem to be a traction force. The dental follicle evolves to produce a complete crown, then the transforming growth hormone is released which attracts osteoclasts and macrophages which cause bone remodelling around the crown. Next, the overlying soft tissue breaks down releasing enamel matrix protein. This may be part of teething – rhinitis, fever, and inflammation of soft tissue around the erupting crown. Growth and thyroid hormones moderate the rate of eruption.

The mechanisms of eruption are not fully understood but factors potentially contributing to eruption are as follows.

• Root formation

Root growth is often happening at the same time as active eruption but seems to follow eruption rather than cause it, e.g. rootless teeth will erupt, and teeth with a closed apex can still erupt.

• Tissue fluid hydrostatic pressure

This mechanism is seen as highly likely. Minute changes in tooth position are synchronized to the pulse and there is a pattern to eruption across the day. Changes in tissue pressure have been recorded corresponding to eruption activity, increased vascularity, and expanding of blood vessels – producing a swollen ground substance.

• Bone remodelling

Bone is certainly remodelled during tooth eruption (e.g. bone tissues can be resorbed locally to make room for the developing clinical crown) but it does not seem to be a major motive force.

• Periodontal ligament

Fibroblasts migrate along the periodontal ligament at the same rate as teeth erupt but this is thought to be a passive process and there seems to be no traction force here.

Tooth eruption dates vary from person to person, and up to 1 year either side of the standard dates should be allowed. See the below diagram for the standard dates for the eruption of baby teeth. Normally, primary teeth fall out and new teeth erupt more-or-less simultaneously, so any 1-sided delay, beyond a few months, should be investigated. Causes for delay are most commonly local obstruction by a supernumerary or impacted tooth or because there is insufficient space for it to erupt into.

Hugh Devlin and Rebecca Craven, used with permission.

Even after the tooth is fully erupted there continues to be an adaptive process of remodelling bone and cementum which ensures that teeth remain in contact and vertical. Forces on teeth, whether continuous or intermittent, during eruption can slow, stop or reverse eruption or redirect its path.

Hugh Devlin is a professor of restorative dentistry at the University of Manchester, where he has been teaching for over 35 years. Rebecca Craven is a senior lecturer in dental public health at the University of Manchester, and worked in general practice, community and hospital dentistry before spending over 25 years in university research and teaching. They are co-authors of the Oxford Handbook of Integrated Dental Biosciences, Second Edition (OUP, 2017).

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